Visualization of soluble carbohydrate distribution in apple fruit flesh utilizing MALDI-TOF MS imaging

To confirm availability of Matrix-assisted laser desorption/ionization time-of-flight (MALDI-TOF) mass spectrometry imaging (MSI) for visualizing distribution of soluble carbohydrates in apple (Malus domestica) fruits a horizontal fruit flesh specimen was cut from a matured 'Fuji' fruit, mounted on a glass slide, lyophilized and then ion intensities of individual soluble carbohydrates were probed around the specimen using a MALDI-TOF MSI apparatus automatically. Contents of soluble carbohydrates in adjacent tissue of the same fruit were also determined using HPLC to compare the distribution of individual carbohydrate based on the ion intensities from MALDI-TOF MSI with those from HPLC. Positive correlation (P <  0.001, R² > 0.95) was confirmed between the concentration of each standard carbohydrate and the relative ion intensity of MALDI-TOF mass spectrometry (MS), and thus it seems possible to use the ion intensity of MALDI-TOF MS for determining the relative concentration of carbohydrates in a sample. Singly charged ions attached with a potassium ion only were detected from the apple fruit specimen when DHB was used as a matrix for MALDI-TOF MSI. Graded increase of sucrose content from center to cortex side of the fruit flesh was confirmed by both MALDI-TOF MSI and HPLC. When pseudo color images on the distribution of individual carbohydrates based on the results from MALDI-TOF MSI were compared with the content of carbohydrates in the adjacent 16 tissue blocks quantified using HPLC, strong (P <  0.001, R² = 0.6222) and weak (P <  0.10, R² = 0.2123) correlation was confirmed between the brightness and the content of sucrose and sorbitol, respectively. These facts indicate that distribution of sucrose and sorbitol in apple fruit tissue can be visualized using MALDI-TOF MSI. Thus, MALDI-TOF MSI will be useful for examining carbohydrate metabolism during the maturing of apple fruit.